1. Field of the Invention
The present invention relates generally to power converters and, more particularly, to a d.c. to d.c. converter that can provide various d.c. output voltages from a single d.c. voltage source.
2. Description of Related Art
Conventional d.c. to d.c. converters are comprised of two major components: a transformer and fast-switching, high-current, high-voltage transistors. The basic operation is to convert the voltage from a d.c. power source to a.c. square wave power at the transformer primary. The square wave power available at the transformer secondary is then rectified to generate a desired d.c. output voltage.
There are two primary type of d.c. to d.c. converters: driven converters and self-oscillating converters:
A conventional driven d.c. to d.c. converter, shown in FIG. 1, is comprised of two transistors Qa, Qb that are alternately switched on by an external square wave drive signal. It can be appreciated that the external square wave drive signal results in square waves of voltage and current at the transistor collectors. In other words, current flows in each transistor Qa, Qb on alternate half cycles of the external square wave drive signal.
The problems that are associated with a conventional driven d.c. to d.c. converter like that shown in FIG. 1 include: (1) bipolar transistors require base drive current that is dissipative; (2) bipolar transistors have saturation losses larger than that associated with MOSFETs; (3) output load regulation is hampered by varying Vce(sat): and (4) MOSFETs can be operated at higher switching frequencies, thereby reducing magnetic size.
FIG. 2 depicts a conventional self-oscillating d.c. to d.c. converter comprised of a d.c. source Vcc, a pair of BJT transistors Qc, Qd, and a transformer Tr1. The primary of transformer Tr1 includes a main center tapped winding NP1:NP2 and an auxiliary center tapped winding NB1:NB2. The transistors Qc, Qd are operated as switches via a feedback connection between the auxiliary base winding NB1:NB2 each transistor base.
The operation of the conventional dc/dc converter is as follows: Assume that transistor Qc begins to turn on as current flows to its base from the DC source Vcc through RB1 and base winding NB1. As transistor Qc turns on and begins to saturate, current from the DC source Vcc begins to flow to ground through the primary winding NP1 and the collector/emitter junction. The voltage impressed on the primary winding NP1 is Vcc-Vce(sat) or, approximately, Vcc-1. A transformer coupling exists between the primary winding NP1 and the base winding NB1. Thus, the voltage induced across NB1 is (NB1/NP1)(Vcc-1).
While current is flowing through the primary winding NP1, the transistor Qc is held on because of the positive voltage induced in the base winding NB1. However, once the transformer core saturates, the voltage across the primary winding NP1 drops to zero, forcing the voltage at the collector of Qc to be forced to near Vcc. If the voltage across the primary winding NP1 collapses, so does the voltage across the base winding NB1.
As all of the winding voltages collapse to zero, Qd is turned partially on as the current from R3 is partially diverted into its base. As current is drawn through the collector of Qd, a voltage starts to appear across primary winding NP2 and, by transformer action, also across base winding NB2. The transistor Qd saturates as the voltage across the base winding NB2 provides additional drive to the base of Qd. As before, the transformer core saturates, the winding voltages collapse, and drive current begins to appear at the base of Qc.
The just-described conventional dc/dc converter is relatively expensive because it requires a transformer having a plurality of primary windings in order to generate the base drive currents. Moreover, relatively durable components are needed because of the large current spikes created when the voltages on the windings of the saturated transformer core collapse. These inherent current spikes induce additional electromagnetic interference. The frequency of oscillation in such a self-oscillating converter is difficult to design and control since it is a function of temperature, component, and voltage supply tolerances. Finally, these conventional circuits require additional "bump start" components, such as a diode D.sub.B and a resistor R.sub.B, to initiate oscillations at power up.